Add unique-names anchor to fragments docs so graphql-tag warning links resolve

The graphql-tag library warns when duplicate fragment names are found
and links to http://dev.apollodata.com/core/fragments.html#unique-names.
This URL no longer resolves. Add a dedicated section with the {#unique-names}
custom anchor ID to the Apollo Client fragments docs page so the link
serves useful content instead of a 404.

Fixes #13292

Author: atharv-sys32

.changeset/quiet-pans-doubt.md

@@ -0,0 +1,5 @@
+---
+"@apollo/client": patch
+---
+
+Add `{#unique-names}` anchor to the fragments docs page so the `graphql-tag` warning about duplicate fragment names links to a valid page.

docs/source/data/fragments.mdx

@@ -41,6 +41,18 @@ query GetPerson {
 
 Changes to the `NameParts` fragment automatically update the fields included in any operations that use it. This reduces the effort required to keep fields consistent across a set of operations.
 
+## Unique fragment names {#unique-names}
+
+Fragment names must be unique across your entire application. If you accidentally define two fragments with the same name but different field selections, `graphql-tag` warns you about the conflict at runtime to prevent subtle bugs:
+
+```text
+Warning: fragment with name SomeFragment already exists.
+graphql-tag enforces all fragment names across your application to be unique; read more about
+this in the docs: http://dev.apollodata.com/core/fragments.html#unique-names
+```
+
+Always use descriptive, component-scoped fragment names (like `ItemFragment` or `UserProfileFragment`) to avoid accidental name collisions.
+
 ## Example usage
 
 Let's say we have a blog application that executes several GraphQL operations related to comments (submitting a comment, fetching a post's comments, etc.). Our application likely has a `Comment` component that is responsible for rendering comment data.